1. Field of the Invention
The invention relates to a process for wet screening of stock suspensions in a pressure grader, and to a pressure grader screen for wet screening of stock suspensions.
2. Discussion of Background Information
An important use of the above-noted process and device is the grading of stock suspensions. In the known process, the fibers contained in the suspension are to pass through the screen, while the undesired solid components are rejected at the slit and conducted out of the screen again. Because the apertures have an essentially elongated shape, e.g., slots or slits, fibrous particles pass through more readily than flat particles, even if both types are present in a similar order of magnitude. With such grading technology, it is therefore possible to achieve a very good separation effect of non-fibrous disruptive substances from stock suspensions. However, it is a prerequisite that the shape of the slot be extremely precise over the entire screen surface. Another important use is the separation of different fibers, in particular fractionation, e.g., based on fiber length or flexibility.
Screens or revolving screens are known, e.g., from published Patent Application DE 33 27 422 A1, in which the grading slots are formed by essentially parallel identical profiles connected to transverse holding ribs. The profiles are shaped and arranged so that, when the screen is in operation, the approach surfaces of the profiles have an oblique contour and a break-away edge behind. This causes the suspension flow sweeping past to undergo a deflection that leads the suspension flow away from the grading slot so that eddies are formed that assist in keeping the grading slots clear and promoting the passage of the fibers through the screen.
In almost all processes of this type, a clearer is moved past close to the screen to produce pressure and suction impulses on one side of the screen. The brief pressure and suction impulses cause the flow direction to reverse in the grading aperture. In this way, the grading aperture is kept clear in that fiber clumps or particles that cannot pass through are conveyed back again against the normal flow-through direction. Pressure and suction impulses can also be introduced into the suspension directly, e.g., via membranes.
The above-mentioned devices have proven to be effective in many applications. However, it is still not always possible to reliably keep the grading slots clear and, at the same time, to conduct the desired high underflow quantity through the screen slots.
In U.S. Pat. No. 4,898,665, a screen device is described in which step-shaped flow barriers ("obstacles") are installed in the inflow area to each screen aperture, which barriers stem the approaching suspension. They also cover the screen apertures partially or completely and produce turbulence in the flow. The eddies thus produced at the end of the flow barrier are intended to guide the flow directly into the screen apertures. However, the flow barriers can also lead to stagnation points on the screen surface at which disruptive deposits settle.